106 BULLETIN OF THE 



aural branch on the anterior extremity of the lateral recalls the same feature 

 in Bhinoptera. Apparently the number of brancblets and openings is greater 

 in Dicerobatus than in either of the other genera cited, and they form closer 

 aggregations along the laterals or over the head. Connection between the 

 laterals, across the vertebral line in the vicinity of the shoulder girdle, has 

 not hitherto been observed. Still greater differences exist in the cephalic 

 canals. If a specimen of one of the species of Rhinoptera were to have the 

 pre-oral fins separated along the median line, and their inner edges carried 

 upward and outward so as to be united to the skull along the edge below the 

 eye, the mouth being at the same time much widened, an arrangement of the 

 canals might be brought about that would present a somewhat near approach 

 to that obtaining in Dicerobatus, so far as the distribution of the main vessels 

 is concerned. The affinities between these genera are well indicated in the 

 canals. 



Laterals . — From the aural each lateral passes obliquely outward to the 

 post-aural branch; thence it takes its way toward the point of junction of 

 shoulder girdle and vertebral column. Nearing the latter, it sends a couple 

 of tubes across it to the lateral of the opposite side, and immediately behind 

 them turns inward and around, under itself, so as to make a rounded loop just 

 in front of the pre-scapular enclosure. This may be an individual peculiarity. 

 Behind the area the pleural is met, and farther back numerous tubules are sent 

 out toward the median line. Half-way to the tail, or farther, some of the 

 tubules pass to the outer side of the canal. In front of the shoulder seven or 

 eight tubules are sent inward toward the vertebrae. The greatest branches are 

 the post-aural and the scapular branch, by which the scapular area is enclosed. 



Pleurals. — Each pleural encloses a branchial area of moderate size, that is 

 widest near the middle of its length and pointed toward each end. Twenty- 

 four branches pass outward from the canal, in the specimen at hand; the 

 median reach little more than half-way from the middle of the back to the 

 tip of the pectoral. The posterior of these tubules are more branched than 

 the anterior, the latter being short, confused, and irregular. To make its 

 descent to the lower surface the pleural passes through the edge of the disk, a 

 short distance behind the spiracle, and drops downward, meeting on the way 

 several tubes connecting with the orbital, until below the level of the eye, 

 where it turns forward nearly parallel with, and a short distance below, the 

 suborbital. With the latter it is connected at narrow intervals by short tubes, 

 a half-dozen or more in number. Below the pleural, in the suborbital region, 

 there are about a dozen short tubules with numerous fine branches, the open- 

 ings of which appear as thickly strewn dots on the surface. Some of these 

 tubules originate in the pleural, the majority, however, belong to the sub- 

 orbital. A little distance in front of the eye the pleural passes obliquely back- 

 ward and inward to the lower surface, making its appearance a very little in 

 front of the nostril. From this its course is somewhat irregular backward and 

 outward to a point below the spiracle, whence it turns still more outward and 

 upward toward the lower side of the pectoral near the anterior border. Oppo- 



